Air ducts of various lengths and sizes are used to transfer clean, filtered air from an air filter through an engine air intake system. It is also well known to use air ducts to transfer cooled or heated air from within the engine compartment to the passenger compartment of an automobile.
Air ducts have been formed by using a rubber molding process. Although the rubber molded air ducts are effective, they are considered too heavy for use in vehicles requiring lighter weight to improve fuel efficiency. Additionally, rubber molded air ducts are bulky and difficult to mold in the serpentine configurations desired for automobiles with limited space in the engine compartment.
An alternative to using a one-piece rubber molded air duct is a three piece construction which utilizes a thermoplastic blow molded tubular body with injection molded rubber seals affixed to each end thereof. Although lighter and more compact than a rubber air duct, the three piece construction has its own inherent problems. The tubular body must be mechanically secured or adhesively bonded to the seals to effect a connection therebetween that can withstand the vibration and heat within an engine compartment. Furthermore the connection between the tubular body and the molded seal tends to break after the air duct has been repeatedly detached and re-attached to the air filter. A complete break in the connection between the tubular body and the seal renders the air duct unusable. Even a slight break between the two parts may allow unwanted impediments to enter the air intake system of the engine. Moreover, the additional manufacturing steps of independently molding the rubber seals and securing the seals to the tubular body adds additional cost to the air duct.
One approach to providing a more secure connection between the tubular body and the rubber seals affixed to each end is disclosed in U.S. Pat. No. 5,266,262 to Narayama et al. Narayama discloses a plurality of blow molded tubes having an interconnecting single flange thickness injection molded around one end of the tubes. More particularly, after the blow molded tube is removed from the mold and cooled, each end of the tube has its inner diameter modified so as to be coaxial with the tube's outer diameter. This method provides a tube with a reliable inner diameter into which a mandrel can be inserted so as to provide an effective seal therebetween during the injection molding operation. The Narayama patent also discloses a method wherein the injection mold is sized to seal around the outer diameter of the blow molded tube.
Although the Narayama patent is an improvement over prior types of air ducts, there are still several limitations. First, the method taught requires an additional manufacturing operation for modifying the diameter of the tube, which increases the cost of the air duct. Moreover, the boring or grinding machine which is employed to modify the inner diameter of the tube is likely to leave frangible pieces of plastic material that may enter the air intake system of the engine. Secondly, the patent reveals that projections are provided on the blow molded tube to more securely couple the injection molded part to the tube. Finally, the positioning of the blow molded tubular body within the injection molding cavity only restrains the movement thereof at two places, around the circumference of the tubular body and at an outwardly extending flange. This positioning permits the tubular body to "float" within the injection mold cavity, thereby allowing dimensional fluctuation of the finished air duct.
Although it is known to provide a rubber molded or combination molded air duct, the prior art does not disclose a method for manufacturing an air duct by injection molding end connecting means or cuffs over a blow molded tubular body that provides a less costly, more reliable interconnection between the two moldings. Furthermore, the prior art does not provide a method for manufacturing an air duct by injection molding end connecting means over a blow molded tube that positively ensures dimensional stability of the finished product. The present invention provides such a method and air duct, thereby increasing the reliability of the air duct manufacturing process and reducing the cost of the finished article.